Vinyl ether terminated urethane resins are extremely reactive prepolymers which are known to undergo rapid polymerization when exposed to radiation. These compounds are particularly useful as curing agents in applications which require high speed curing of a resin formulation, such as in radiation curable coatings.
One disadvantage attendant to the use of such vinyl ether functionalized urethanes is that their commercial availability is relatively limited. In general, the available prepolymers constitute liquid or semi-solid (with extremely low Tg .degree.C.) materials. U.S. Pat. No. 4,751,273 (Lapin, et al.) provides specific examples of such liquid and semi-solid vinyl ether terminated urethane resins.
These curing agents, though extremely useful in liquid radiation curable coatings, have only limited use in powder coatings. Typically, because of their liquid or semi-solid state, they cannot be used beyond a few percent (&lt;5%) in powder coatings. Greater amounts typically cause the powder to block or sinter in storage, which renders the powder unsprayable during electrostatic coating operations.
Solid vinyl ether terminated urethane curing agents which are more conducive for use in radiation curable powder coatings have been proposed. For example, EP-A-0 636 669 (DSM, N.V.) provides one example of a crystalline vinyl ether functionalized urethane curing agent that remains a solid at room temperature (melt range of 90-108.degree. C.). This curing agent arises from the reaction of hydroxybutyl vinyl ether (HBVE) with hexamethylene diisocyanate (HDI) monomer in a 1:1 (stoichiometric) mole ratio of hydroxy to isocyanate groups. The reaction product is a short chain crystalline urethane oligomer (HBVE-HDI-HBVE).
One disadvantage with the use of such a crystalline curing agent in powder coatings is that it makes manufacture of the powders extremely troublesome. Powders based on crystalline materials take longer to recrystallize after melt extrusion, making subsequent grinding and handling very messy and difficult.
Another disadvantage with the use of this curing agent is that monomeric HDI is known to be unsafe to handle because of its high toxicity. Thus, the presence of residual (unreacted) monomeric HDI in the curing agent will expose the end user to serious health hazards. For instance, HDI monomer has been known to cause skin sensitization, which can lead to serious respiratory disease in workers, including asthma and permanent decrease in lung functions. Furthermore, HDI monomer readily becomes airborne because of its high vapor pressure at room temperature which, in turn, increases the risk of inhalation of its vapors or mists.
It would be desirable to provide a vinyl ether terminated urethane curing agent that is solid at room temperature, is easier to melt process, is much safer to handle, and is effective in curing powder coatings.